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United States Patent |
5,349,194
|
Wuest
,   et al.
|
September 20, 1994
|
Microgap ultra-violet detector
Abstract
A microgap ultra-violet detector of photons with wavelengths less than 400
run (4000 Angstroms) which comprises an anode and a cathode separated by a
gas-filled gap and having an electric field placed across the gap. Either
the anode or the cathode is semi-transparent to UV light. Upon a UV photon
striking the cathode an electron is expelled and accelerated across the
gap by the electric field causing interactions with other electrons to
create an electron avalanche which contacts the anode. The electron
avalanche is detected and converted to an output pulse.
Inventors:
|
Wuest; Craig R. (Danville, CA);
Bionta; Richard M. (Livermore, CA)
|
Assignee:
|
The United States of America as represented by the United States (Washington, DC)
|
Appl. No.:
|
011636 |
Filed:
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February 1, 1993 |
Current U.S. Class: |
250/372; 250/374 |
Intern'l Class: |
G01J 005/28; H01J 040/06 |
Field of Search: |
250/374,372
|
References Cited
U.S. Patent Documents
3342995 | Sep., 1967 | Axmark | 250/372.
|
3656019 | Apr., 1972 | Stowe | 313/217.
|
3952197 | Apr., 1976 | Samson | 250/382.
|
3965354 | Jun., 1976 | Fletcher et al. | 250/336.
|
4376892 | Mar., 1983 | Charpak et al. | 250/372.
|
4614871 | Sep., 1986 | Driscoll | 250/372.
|
4871915 | Oct., 1989 | Prince | 250/372.
|
4889994 | Dec., 1989 | Brown et al. | 250/374.
|
5001348 | Mar., 1991 | Dirscherl et al. | 250/372.
|
5032729 | Jul., 1991 | Charpak | 250/385.
|
Foreign Patent Documents |
91/15028 | Oct., 1991 | WO | 250/374.
|
Other References
A. Peisert and F. Sauli, "A Two-Dimensional Parallel-Plate Chamber for
High-Rate Soft X-Ray Detector", Nuclear Instruments and Methods In Physics
Research A, vol. 127 (Jul. 1, 1986) pp. 453-459.
|
Primary Examiner: Hannaher; Constantine
Attorney, Agent or Firm: Carnahan; L. E., Gaither; Roger S., Moser; William R.
Goverment Interests
The United States Government has rights in this invention pursuant to
Contract No. W-7405-ENG-48 between the U.S. Department of Energy and the
University of California for the operation of Lawrence Livermore National
Laboratory.
Claims
We claim:
1. A detector for ultra-violet photons comprising:
a cathode composed of material that expels electrons when impinged on by
ultra-violet photons;
an anode composed of material that collects electrons;
one of said cathode and anode being constructed of a layer of material
which is semitransparent to ultra-violet photons;
said layer of material semitransparent to ultra-violet photons extending
over an entire surface of said one of said cathode and anode constructed
thereof;
a window composed of material transparent to ultra-violet photons;
one of said cathode and anode being located adjacent to said window;
aid cathode and said anode being located in spaced relation to form a gap
therebetween;
said gap being filled with a gas;
means for applying an electric potential between said anode and said
cathode for producing an electrical field in said gap; and
means for detecting electron avalanches on said anode.
2. The detector of claim 1, wherein said anode is constructed of material
sen-ti-transparent to ultra-violet photons.
3. The detector of claim 2, wherein said anode material is selected from
the group consisting of gold, copper, silver and platinum.
4. The detector of claim 3, wherein said anode material is gold having a
thickness of about 200 .ANG..
5. The detector of claim 2, wherein said cathode is constructed of material
selected from the group of yttrium, cesium iodide, gold, silver and
chromium.
6. The detector of claim 5, wherein said cathode is constructed of yttrium
having a thickness of about 1000 .ANG..
7. The detector of claim 1, wherein said gap has a width of 5 .mu.m to 1
mm.
8. The detector of claim 7, wherein said gap has a width of about 100
microns.
9. The detector of claim 1, wherein said gas has a pressure in the range of
0.1 atm to 2 atm, and is selected from the group consisting of 90%
argon/10% methane, argon/isobutane mixtures, and CO.sub.2 /CF.sub.4.
10. The detector of claim 9, wherein said gas is composed of 90% argon/10%
methane at a pressure of 1 atm.
11. The detector of claim 1, wherein said electric field, E, is determined
by the electric potential, V, applied between said anode and said cathode
divided by the dimension, D, of said gap, namely, E=V/D.
12. The detector of claim 11, wherein said electric potential, V, is 500
volts and said dimension, D, is 100 microns, whereby said electric field
is 5.times.10.sup.6 volts/meter.
13. The detector of claim 1, wherein said cathode is constructed of
material semi-transparent to ultra-violet photons.
14. The detector of claim 13, wherein said anode is constructed of opaque
metallic material selected from the group consisting of gold, silver,
copper, chromium and platinum.
15. The detector of claim 13, wherein said cathode is constructed of
material selected from the group consisting of yttrium, cesium iodide,
gold, silver and chromium.
16. The detector of claim 15, wherein said cathode is constructed of
semi-transparent yttrium having a thickness of 200 .ANG..
17. The detector of claim 16, wherein said anode is constructed of gold.
18. The detector of claim 17, wherein said gold anode has a thickness of
1000 .ANG..
19. A micro-gap ultra-violet detector comprising:
a photocathode;
an anode;
one of said photocathode and said anode being constructed of material which
is semi-transparent to ultra-violet photons;
said photocathode and said anode being located in coplanar spaced
relationship along all surfaces thereof an forming a gap therebetween;
said gap being filled with gas;
means for applying an electrical potential between said anode and said
photocathode for producing an electric field in said gap; and
means for detecting and removing a charge on said anode created by a photon
striking said photocathode.
20. The detector of claim 19, wherein said anode is constructed of
semi-transparent material.
21. The detector of claim 19, wherein said photocathode is constructed of
semi-transparent material.
22. A micro-gap detector for detecting photons with wavelengths less than
400 nm, comprising:
a housing having an opening therein;
a photocathode and an anode being located in coplanar spaced relationship
at all points thereon, and located in said opening of said housing and
forming a gap therebetween;
one of said coplanar photocathode and anode being composed of material
which is semi-transparent to ultra-violet photons;
said gap being filled with a gas;
means for producing an electric filed in said gap; and
means for detecting and removing electrons collected on said anode.
23. A micro-gap detector for detecting photons with wavelengths less than
400 nm, comprising:
a housing having an opening therein;
a photocathode and an anode being located in coplanar spaced relationship,
and located in said opening of said housing and forming a gap
therebetween;
one of said coplanar photocathode and anode being composed of material
which is semi-transparent to ultra-violet photons;
said gap being filled with a gas;
means for producing an electric field in said gap; and
means for detecting and removing electron collected on said anode;
each of said photocathode and said anode being configured to have a width
and length, said photocathode having a width dependent upon the
configuration of the anode, and said anode being configured in differing
widths when selected from the group consisting of a continuous plane, and
a plurality of sections.
Description
BACKGROUND OF THE INVENTION
The invention relates to photon detectors, particularly to an ultraviolet
detector which involves the conversion of ultra-violet photons into
electrons and subsequent amplification of these electrons via generation
of electron avalanches.
Photon detectors operate by converting photons into electronic signals that
can be processed into pulses or images. These include devices such as
photodiodes, photomultiplier tubes, vidicons, charged-coupled devices
(CCD's) etc. All photon detectors are characterized by their sensitivity
to photons as a function of photon energy, their ability to amplify
incident photons into large electrical signals proportional to the
incident photon intensity (gain), their ability to distinguish fine detail
in an image (position resolution), their temporal response to incident
photons (time resolution), and their inherent noise level (dark current).
Various types of photon detectors and detection systems have been developed
for various applications. These prior detectors and detection/imaging
systems are exemplified by the following U.S. Pat. No. 5,032,729 issued
Jul. 16, 1991 to G. Charpak; U.S. Pat. No. 5,001,348 issued Mar. 19, 1991
to R. Dirscherl et al.; U.S. Pat. No. 4,889,994 issued Dec. 26, 1989 to A.
R. Brown et al.; U.S. Pat. No. 4,871,915 issued Oct. 3, 1989 to K. C.
Prince; U.S. Pat. No. 4,614,871 issued Sep. 30, 1986 to J. N. Driscoll;
U.S. Pat. No. 3,965,354 issued Jun. 22, 1976 to J. C. Fletcher et al.;
U.S. Pat. No. 3,952,197 issued Apr. 20, 1976 to J. A. R. Samson; U.S. Pat.
No. 3,656,019 issued Apr. 11, 1972 to R. W. Storve and U.S. Pat. No.
3,342,995 issued Sep. 19, 1967 to R. E. Axmark.
Imaging in high radiation environments, such as in nuclear reactors, normal
ultra-violet (UV) sensitive CCD cameras and vidicon TV cameras are plagued
by noise pickup. Also, the prior known systems are not greatly effective
when used in extremely low light level sensitivity television cameras.
While photomultiplier tubes are widely utilized in the field of
ultra-violet radiation detection they are expensive and susceptible to
magnetic field.
The Superconducting Super Collider, when developed will require
scintillation counters, and sets forth a need to instrument roughly 6000
square meters of liquid or solid scintillator with photodetectors that are
rugged, radiation-hardened, and able to operate in a high magnetic field.
This need is not easily fulfilled from a operation and cost effective
standpoint by the prior known photomultiplier tubes due to their cost and
susceptibility to magnetic fields. Thus, there is a need for a simple yet
effective ultra-violet sensitive detector.
The above need is satisfied by the microgap ultra-violet detector of this
invention which is of simple construction, exhibits photon sensitivity
over a wide range of energies, exhibits fast time response, provides
adjustable gain, and exhibits low noise. Unlike CCD detectors, the present
invention is not subject to radiation damage.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an improved
ultra-violet detector.
A further object of the invention is to provide a detector of photons with
wavelengths less than 400 nm.
A further object of the invention is to provide a detector for ultraviolet
photons wherein the photons are converted to electrons with subsequent
amplification through generation of electron avalanches which are detected
and converted to output signals.
A further object of the invention is to provide an ultra-violet detector
which exhibits photon sensitivity over a wide range of energies, fast time
response, adjustable gain, and low noise.
Another object of the invention is to provide a detector for ultraviolet
photons which is simple in construction in that it consists of a coplanar
anode and cathode separated by a thin gas-filled gap having an electrical
potential applied there across and provided with charge sensing means for
detecting electron avalanches on the anode.
Another object of the invention is to provide a microgap ultraviolet photon
detector wherein either the anode or the cathode is semitransparent to
ultra-violet light.
Other objects and advantages of the present invention will become apparent
from the following description and accompanying drawings.
The invention is directed to a microgap ultra-violet (UV) detector for
detecting photons with wavelengths less than 400 nm (4000 Angstroms). The
microgap detector comprises an anode and a cathode separated by a gap.
Either the anode or the cathode is semi-transparent to UV light. The
cathode operates as a photocathode that expels electrons when UV photons
are incident thereon. The gap between the anode is filled with a gas and
an electric potential is applied between the anode and cathode to produce
an electric field within the gap. When electrons are expelled from the
cathode they are accelerated across the gap by the electric field and
interact with other electrons in the gas resulting in the generation of an
electron avalanche which is directed onto the anode and detected by a
charge sensitive preamplifier which converts to electron charge into an
output pulse, which is directed through conventional electronic circuitry
to a point of use, such as an oscilloscope or a camera.
The microgap ultra-violet detector of the present invention may be utilized
in a wide variety of applications such as detecting emissions from
combustion products, in scintillation counters, imaging detectors for
fiber optic bundles, with variable light levels, in high radiation
environments such as nuclear reactors having a high noise level, for
extremely low light sensitivity cameras, and nearly all applications where
UV-sensitive CCD cameras are currently used.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated into and form a part of
the disclosure, illustrate embodiments of the invention and, together with
the description, serve to explain the principles of the invention.
FIG. 1 is a schematic view of an embodiment of the microgap ultra-violet
detector with a semi-transparent anode and an opaque photocathode.
FIG. 2 is a schematic view of another embodiment of the microgap
ultra-violet detector with a semi-transparent photocathode and an opaque
anode.
DETAILED DESCRIPTION OF THE INVENTION
The microgap ultra-violet (UV) detector of the present invention achieves
the conversion of ultra-violet photons into electrons (photoelectrons) and
subsequent amplification of these photoelectrons through the generation of
electron avalanches in a thin gas-filled region or gap, between the anode
and the cathode, which is subjected to a high electric potential. The
detector of this invention has a number of desirable features that makes
it a potentially excellent replacement for present-day photodetectors. The
microgap UV detector exhibits photon sensitivity over a wide range of
energies, with an upper limit of about 8 eV (150 nm wavelength) and a
lower limit of about 3.1 eV (400 nm wavelength). The detector exhibits the
fast time response typical of photomultiplier tubes, with pulse widths
less than 10 nanoseconds (ns) and as fast as 1 ns, thus allowing it to
operate with frequency response up to the gigahertz (GHz) level. The
detector provides adjustable gain of up to about 10.sup.9 depending on the
choice of fill gas and electric potential. Also, the detector exhibits low
noise, typical of photomultiplier tubes.
Suitable choices of the photocathode and anode geometry allow this detector
to perform as a position sensitive detector, with position resolution
similar to a CCD or Vidicon, but with a factor of 1000 times the speed of
readout. The speed of the detector allows for unprecedented dynamic range
of visual information, allowing for the detection of photons with high
fidelity in conditions that would normally saturate CCD detector pixels.
This dynamic range enhancement is achieved through a combination of the
inherent time response of the detector and the application of fast
electronics as read out individual anode elements in a parallel manner.
While not illustrated herein, the UV detector may be electronically
connected as a microgap camera similar to the microgap x-ray camera
arrangement illustrated in applicants' copending U.S. patent application
Ser. No. 08/011,637, filed Feb. 1, 1993, entitled "Microgap X-ray
Detector", and assigned to the same assignee.
The microgap ultra-violet detector of the present invention is
schematically illustrated by the embodiments of FIGS. 1 and 2. In it
simplest form the detector consists of a coplanar anode and cathode
separated by a thin gas-filled gap with a width or dimension, D. An
electrical potential, V, is applied between the anode and the cathode to
provide an electric field, E, that is equal to the electric potential
divided by the gap dimension, E=V/D. The gap dimension and electric
potential are chosen in order to provide an electric field of the order of
10.sup.6 volts per meter. Thus, a gap of 100 microns (10.sup.-4 m) and an
electric potential of 500 volts gives an electric field, E=500
volts/10.sup.-4 m=5.times.10.sup.6 volts/meter.
The embodiment of FIG. 1 comprises a housing 10 constructed of aluminum
having an opening 11 extending there through within which are located a
window 12 constructed of quartz, an anode 13 constructed of 200 .ANG.
semi-transparent gold, a cathode (photocathode) 14 constructed of yttrium,
with a gap or region 15 having a width of 100 microns between anode 13 and
cathode 14 and containing a gas 16 composed of 90% argon/10% methane,
known as P10 and at a pressure of 1 atm. A power supply 17 is operatively
connected as indicated by dashed lines to the anode 13 and cathode 14 for
directing an electric potential of 500 volts there between for producing
an electric field through gas 16. A charge detector 18, such as a charge
sensing pre-amplifier, is electronically connected as indicated by dashed
line to detect charges produced by electron avalanches on the anode 13 and
convert such charge pulses into a current or voltage pulses that can be
read using standard electronic circuitry.
The housing 12, while described above as being constructed of aluminum, may
also be made of plastic, ceramic or stainless steel, with the opening 11
therein having a length of 20 cm and width of 20 cm and height of 5 cm,
for example. The window 12, described above as being constructed of quartz
and has a thickness of 2 ram, may also be made of fused silica, UV
transparent plastic, and sapphire, or other material transparent to
ultra-violet photons, and having a thickness of 1 mm to 5 mm, depending on
the type of material it is made of.
Cathode 14 consists of a material with a property such that it expels
electrons when photons are incident upon it. Cathodes of this type are
called photocathodes and the expelled electrons are called photoelectrons.
The photocathode 14 of this detector is sensitive to ultra-violet light
and in the FIG. 1 embodiment consists of yttrium, which was chosen because
of its UV-sensitivity as well as its stability (lack of oxidation and
degradation in the presence of the gas 16). However, the photocathode 14
may also be constructed of cesium iodide, silver, gold and chromium. By
way of example, the photocathode 14 constructed of yttrium in the FIG. 1
embodiment has a cross-section of 6 cm.times.6 cm and thickness of 100
.mu.m. The thickness of the photocathode will be dependent upon the
material thereof and the width will be dependent upon the configuration of
the anode 13, be it a continuing plane or a plurality of spaced sections,
which may result in differing widths.
When a photoelectron is ejected out of the photocathode 14 and into the
thin gas-filled gap 15 it is accelerated in the extremely high electric
field within gap 15 towards the anode 13. The accelerating electron
collides with other electrons in the atones of gas 16 to create additional
electrons, and so on, until an electron avalanche is formed. An electron
avalanche can best be thought of as a sort of electron chain reaction.
Under the proper choices of gas type, and pressure, and electric field,
gains of 10.sup.9 or more can be achieved by the time the electron
avalanche reaches the anode 13. The avalanche is detected on the anode 13
with a detector 18, such as a charge sensing preamplifier, known in the
art, which converts the charge pulse into a current or voltage pulse or
signal that can be read via standard electronic circuitry.
While the gap 15 in the FIG. 1 embodiment has been indicated above as
having a width or dimension, D, of 100 microns, the gap width may vary
from 5 .mu.m to 1 mm, depending on the type of gas 16 and the electric
potential applied there through. While the gas 16 has been described above
as being composed of 90% argon/10% methane, it can be argon/isobutane
mixtures, or CO.sub.2 /CF.sub.4, and the pressure may vary from 0.1 atm to
2 atm depending on the type of gas and the desired electric field there
through. Also, the electric potential provided by power supply 17, depends
on the desired electrical field, gas type, and pressure, and may vary from
the 500 volts as set forth in the above description of the FIG. 1
embodiment.
The anode 13 is configured as a conducting plane, consisting of a thin 200
.ANG. (20 nm) coating of gold on window 12. This gold coating is
semitransparent to UV photons, allowing them to pass there through to the
photocathode 14 and providing an electrode surface on which electron
avalanches can be collected. In this case, the microgap ultra-violet
detector is analogous to a photomultiplier tube or photodiode. Anode 13
may also be constructed from other materials such as silver, chromium,
copper and platinum, which are semi-transparent to UV photons but will
function as an electrode for collecting the electron avalanches, and the
thickness of the anode will depend on the material from which it is
constructed. While anode 13 is described above as being configured as a
conducting plane, it can be configured in spaced sections or as a square,
for example, so as to function as a position sensitive detector, with
position resolution similar to a CCD or Vidicon.
The microgap ultra-violet detector of this invention may also utilize a
semi-transparent photocathode, in which case the anode is opaque. A
detector embodiment of this type is illustrated in FIG. 2, and reference
numerals corresponding to the components of the FIG. 1 embodiment are
used. In this embodiment the semi-transparent photocathode 14 is coated on
window 12. The operation of the detector embodiment of FIG. 2 is basically
unchanged compared to the FIG. 1 embodiment, except that certain of the UV
photons pass through semi-transparent photocathode 14' without causing a
photoelectron to be emitted there from and amplified as above described in
gap 15 such that it generates an electron avalanche which is collected by
the opaque anode 13'. Thus, not all of the photons cause electrons to be
expelled from the photocathode 14'. Anode 13' is constructed of opaque
metallic material such as gold, silver, copper, and platinum, having a
thickness of 500 .ANG. to 1000 .ANG.. In the FIG. 2 embodiment, opaque
anode 13' is constructed of gold having a thickness of 1000 .ANG.; while
semi-transparent photocathode 14' is constructed of yttrium having a
thickness of 200 .ANG.. Semitransparent photocathode 14' may also be
constructed of cesium iodide, gold, and chromium having a thickness
dependent upon the material of which it is constructed.
While the FIG. 2 embodiment illustrates a single anode 13', when multiple
anodes consisting of separate areas of conductive material are used and
laid out in an area, and read out independently of each other, the
individual avalanches can be read on individual anodes using multiple
charge detectors so as to give position information corresponding to the
anode position.
As set forth above, the microgap ultra-violet detector of this invention
has many applications, such as to detect UV emissions from combustion
products, for example, from rocket plumes. Additional applications of the
detector would be for scintillation counters planned to be used in the
Superconducting Super Collider (SSC), as well as imaging detectors for
fiber optics bundles in the SSC and in underground nuclear tests. The
detector can be used in nearly all applications where UV-sensitive CCD
cameras are used including medical imaging. Since the detector exhibits a
high dynamic range it can resolve images with highly variable light
levels, such as an astronomy and astrophysics. Imaging in high radiation
environments which involves inherent noise can be accomplished by the
detector of this invention, as well as for use with extremely low light
level sensitivity television cameras. Because of the simplicity of the
detector of this invention, it is not subject to x-ray or ionizing
radiation damage and has very low sensitivity to spurious noise induced by
background charged and neutral particle radiation.
While particular embodiments of the invention have been illustrated and
described and specific materials, parameters, etc. have been set forth to
more fully illustrate the principles of this invention, such are not
intended to limit the invention to that described and/or illustrated.
Modifications and changes will become apparent to those skilled in this
art. All modification, changes, etc. are within the scope of this
invention when such fall within the scope of the appended claims.
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